Hitherto, as a ball plug valve, there has been one disclosed in Japanese Examined Utility Model Publication No. 3-29656 wherein, between an inlet passage and an outlet passage of a valve body, there is a ball which has a flow through hole and sealing members respectively in contact with the ball on both opening ends thereof, a control section and the ball are connected via a valve stem, and an opening on the inlet end of the flow through hole is circular, while the opening on the outlet end is sectorial.
According to the ball plug valve, the valve can be fully opened or closed by rotating the ball 90 degrees, permitting quick and easy operation, whereas a handle has to be turned several times or a few tens of times as in the case of a sluice valve. The two sides intersecting at the tip of the sectorial opening are shaped into a convex curve which bulges toward the centerline of the opening so that the flow characteristic exhibits an equal percent characteristic (the increasing flow percentage is equal to the same increment as valve opening). Likewise, the aforesaid two sides are shaped into a sectorial curve with respect to the centerline of the opening so that the flow characteristic exhibits a linear characteristic (the flow rate is changed in direct proportion to the opening of the valve) (Japanese Un-Examined Patent Publication No. 55-107165).
In the prior art, however, it is set such that the tip of the sectorial opening is in a sealing region which is formed by a sealing member on the outlet end which is still far away from an outlet passage when the valve is fully closed, whereas the tip of the sectorial opening is still in the outlet passage when the valve is fully open. This has been posing problems 1 , 2, and 3 below:
1 When opening the valve through the control section, it takes time for a fluid to start flowing because of a long floating distance in which the tip of the sectorial opening passes by the sealing members and reaches the outlet passage.
2 When the valve is fully open, since the tip of the sectorial opening is still in the outlet passage, the effective rotational angle range of the ball which connects the sectorial opening to the outlet passage is relative small, thus limiting the band wherein the fluid is controlled through the control section.
3 An electric actuator could be used for the control section to expand the fluid control band wherein input signals issued through the control section are effective; however, the resolution of the control section would be deteriorated accordingly, resulting in deteriorated fluid control accuracy.